WASHINGTON, D.C. — U.S. taxpayers have spent $1.5 billion to allow Union Pacific Corp.’s tracks between Chicago and St. Louis to carry faster passenger trains, one of the biggest projects in President Barack Obama’s high-speed rail program. Now a panel advising the same agency that oversaw the work may require it to be re-done so trains can go even faster, to the potential benefit of Caterpillar Inc. and Siemens AG (SIE) and the detriment of General Electric Co.